This invention relates to a novel process for molding superplastic metals and apparatus useful in carrying out said process.
Superplastic metals are known in the art as materials that provide some of the advantages of both metals and plastics. Although there are a substantial number of metal alloys that exhibit superplastic character under some conditions, some superplastic metals are readily susceptible to commercial exploitation as such. Among these are those such as the zinc alloys available commercially from New Jersey Zinc Company and others. In this invention, the term superplastic metal is limited to those useful in molding operations in which the superplastic properties are utilized. It has been known to mold various articles from such alloys, and it has been particularly advantageous to mold complex forms from such alloys. Thus, for example, such objects as print wheels and metal molds to be used in subsequent injection molding of plastics are known uses for superplastic alloys.
One relatively undesirable feature of the superplastic alloys has been the time required for the molding cycle. This molding cycle time reflects the fact that superplastic materials exhibit substantial strain rate sensitivity and must be formed at carefully controlled rates. Even after the molded item is formed, it is important to exert extreme care in getting it out of the mold. For example, in U.S. Pat. No. 3,578,511, heating the superplastic metal above the critical superplastic forming temperature is recommended as a way to impart a molded article sufficient increased strength to enable its immediate removal from the die in which it has been formed. However, the necessity of heating the metal above the molding temperature is itself a time-consuming and an energy-consuming procedure.
Hence it has remained a problem to obtain rapid economic cycling of molding operations in which articles of superplastic metals are formed.